The present invention relates to a hydraulic rack and pinion power steering system including a control valve having an inner valve member (valve core) which is coaxial with and rotatable relative to an outer valve member (valve sleeve).
In this type of steering system, the inner valve member is formed on a part of a cylindrical input shaft which is connected for rotation with the vehicle steering wheel. The outer valve member is connected for rotation with a follow-up member in the form of a pinion. The pinion is in meshing engagement with the toothed portion of a rack. The rack is drivingly connected with a piston-cylinder type power steering motor and steerable vehicle wheels.
To effect actuation of the power steering motor to turn the steerable vehicle wheels, the inner valve member is rotated relative to the outer valve member, from a neutral condition, for a few degrees against the bias of a torsion bar. The torsion bar is press fitted at one end to the pinion and at the other end is connected by a pinned connection to the input shaft. When the steering wheel is rotated, relative rotation between the inner and outer valve members ports fluid under pressure to one side or the other of the steering motor to effect steering movement of the steerable vehicle wheels.
It is important that the control valve be balanced, so that the same amount of steering assist is generated when the inner valve member is rotated by the same angular amount in either direction from the neutral condition. Typically, the control valve is balanced prior to the torsion bar being pinned to the inner valve member. The operation of pinning the torsion bar to the input shaft, after the valve has been balanced, may cause the control valve to become unbalanced.